1. Field of the Invention
The present invention relates to a plasma treatment apparatus, and more particularly to a plasma treatment apparatus utilizing the electron cyclotron resonance (referred to as ECR hereinafter) discharge.
2. Description of the Background Art
In manufacturing of a semiconductor device such as an IC (Integrated Circuit), treatment such as formation of thin film and etching are applied on the surface of a semiconductor substrate (wafer). Lately, a plasma treatment apparatus using plasma generated by ECR discharge has been developed and put into practical use as a apparatus for processing such a semiconductor substrate. A structure of a conventional plasma treatment apparatus using plasma generated by ECR discharge will be described below.
FIG. 12 is a sectional view schematically showing a structure of a conventional plasma treatment apparatus. Referring to FIG. 12, a plasma treatment apparatus 310 includes a reaction chamber 101, an electromagnetic coil 103, a microwave generating source 104, a waveguide 105, a microwave introducing window 106, a pipe 107, an exhaust hole 108, and a sample table 109.
One end of waveguide 105 is attached to the upper portion of reaction chamber 101. Waveguide 105 and reaction chamber 101 are separated by microwave introducing window 106. The other end of waveguide 105 is attached to microwave generating source 104.
Electromagnetic coil 103 is provided to surround the periphery of reaction chamber 101. Pipe 107 which supplies a reactive gas is provided at the upper portion of reaction chamber 101. Exhaust hole 108 is provided at the bottom of reaction chamber 101. A pump (not shown) is attached to exhaust hole 108. Sample table 109 is attached inside reaction chamber 101. A wafer 120 can be placed on sample table 109.
A conventional plasma treatment method will be described.
Referring to FIG. 12, a residual gas in reaction chamber 101 is well exhausted from exhaust hole 108. Then a reactive gas is introduced into reaction chamber 101 through pipe 107. While the reactive gas is introduced, a portion of the reactive gas is exhausted through exhaust hole 108, so that gas pressure in reaction chamber 101 will be kept at a predetermined value.
Microwave is then generated from microwave generating source 104. The microwave will be introduced into reaction chamber 101 through waveguide 105 and microwave introducing window 106. Meanwhile, a magnetic field, which gradually attenuates from the upper portion of reaction chamber 101 to wafer 120, will be formed in reaction chamber 101 by rendering electromagnetic coil 103 conductive. Plasma is produced by the electromagnetic field and the microwave. Electrons in the plasma move around a magnetic line of force by the Lorentz's force due to a magnetic field. By adjusting the strength of the magnetic field so that a frequency of the circular motion and the frequency of the microwave will be coincided (that is, adjusting magnetic flux density B such that it satisfies f=.vertline.q.vertline..multidot.B/2 .pi.m for the microwave frequency f), energy of the microwave will be converted effectively into kinetic energy of electrons by resonance absorption. This is referred to as electron cyclotron resonance. Usually, the microwave having the frequency of 2.45 GHz, which is a frequency used in the industry, is often used, while the resonance magnetic field of 875 Gauss is often used.
Electrons in the reactive gas in reaction chamber 101 are accelerated by absorbing the microwave energy and move circularly at a high speed. The electrons making circular motion at a high speed collide with the reactive gas molecule, so that the reactive gas in reaction chamber 101 will be ionized at a high ionization ratio for generating a high density gas plasma. The electrons in the plasma are restricted by the magnetic field and move spirally around a magnetic line of force: its momentum will be changed in the direction of the magnetic line of force by the attenuating field, and they travel to wafer 120.
An electric field, so called an ion sheath electric field, which is perpendicular to the surface of wafer 120, will be generated at the surface of wafer 120 by the travel of the electrons. In the ion sheath electric field, plasma side is made positive and the surface of wafer 120 side is made negative. Reactive ions in the plasma, which are positive ions, are accelerated in the direction to wafer 120 by the ion sheath electric field. The reactive ions are incident upon the surface of wafer 120, and thus, treatment such as etching will be effected on the surface of wafer 120 with these ions.
The conventional plasma treatment apparatus 310 using such ECR discharge has such characteristics that the microwave energy are absorbed effectively in electrons, and that electrons are hard to dissipate in the radial direction of reaction chamber 101 due to the magnetic field. Accordingly, a high density plasma can be produced even in a low gas pressure environment where it is difficult to maintain plasma. Thus, the conventional plasma treatment apparatus 310 using ECR discharge is broadly used currently.
Generally, however, the velocity of the thermal motion of electrons in the plasma produced in reaction chamber 101 is sufficiently higher than that of ions. In other words, mobility of electrons in the plasma is greater than ions. Thus the electrons reach the inner wall of reaction chamber 101 earlier than ions, and vanish. Accordingly, in the center of reaction chamber 101 (a dash-dotted line c--c), relatively large number of ions are left behind, and the plasma potential at ECR plane 130 in the radial direction of reaction chamber 101 will be as shown in FIG. 13, where ECR plane means a plane having a magnetic flux density B which satisfies f=.vertline.q.vertline..multidot.B/2 .pi.m for the microwave frequency f, while q denotes the amount of electron charge, and m denotes electron mass.
Referring to FIG. 13, abscissa denotes a position of reaction chamber 101 in radial direction, and ordinate denotes plasma potential. The plasma potential becomes higher positive potential toward the direction of the arrow. The plasma potential indicates the highest positive potential in the center of reaction chamber 101 because relatively large number of ions having positive charges are left behind, and the potential becomes lower toward the inner wall of reaction chamber 101. Meanwhile, an ion sheath region is formed in the vicinity of the inner wall of reaction chamber 101 since the electromobility of electrons is greater than ions, thus the plasma potential in this region drops abruptly. The plasma potential at ECR plane 130 is thus become non-uniform. The above plasma potential being non-uniform in the radial direction of reaction chamber 101 is described in Howe: J.A.P. 24 (1953) 892.
According to the Boltzman relationship, a distribution of electron density n.sub.e in the radial direction of reaction chamber 101 is represented as: EQU n.sub.e (x)=n.sub.e (0) exp (e.phi.(x)/k.sub.B T.sub.e)
where x: distance from the center of reaction chamber, n.sub.e (x): electron density at point x, .phi. (x): a potential at point x (.phi.(0)=0), k.sub.B : Boltzman constant, T.sub.e : electron temperature, and e: charge of one electron.
According to energy conservation equation and its subsequent equation, a distribution of ion density n.sub.i in the radial direction of reaction chamber 101 is represented as: ##EQU1## where M: ion mass and v: average velocity of ions.
Accordingly, when the distribution of plasma potential in the radial direction of reaction chamber 101 is such as shown in FIG. 13, both the distribution of electron density (n.sub.e) and the distribution of ion density (n.sub.i) are decreased from the center to the inner wall of reaction chamber 101 as shown in FIG. 14.
In conventional plasma treatment apparatus 310 utilizing ECR discharge shown in FIG. 12, the distributions of plasma potential, electron density and ion density in the radial direction of reaction chamber 101 are not uniform. Thus, conventional plasma treatment apparatus 310 has the following disadvantages.
FIG. 15A is a schematic diagram showing magnetic lines of force developed within the reaction chamber. FIGS. 15B and 15C are schematic diagrams respectively showing electrons at P2 and Q2 in FIG. 15A. Referring to FIG. 15A, a magnetic line of force 140 which diverges from the upper portion of reaction chamber 101 to wafer 120 is formed in reaction chamber 101 by rendering an electromagnetic coil conductive (not shown). Electrons in plasma in reaction chamber 101 travel along magnetic line of force 140 from the upper portion of reaction chamber 101 to wafer 120. The electron density in the ECR plane in reaction chamber 101 is high in the center of reaction chamber 101, while it is low at the periphery. Thus, in the ECR plane there are more electrons in the center of the reaction chamber 101 than at the periphery. Thus, there are more electrons which travel along magnetic line of force 140p formed in the center of reaction chamber 101 than those which travel along magnetic line of force 140q formed at the periphery. In other words, as shown in FIGS. 15B and 15C, the number of electrons traveling along magnetic line of force 140p at P.sub.2 is greater than the number of electrons traveling along magnetic line of force 140q at Q.sub.2.
Thus, the number of electrons traveling along magnetic line of force 140 in the center of reaction chamber 101 is not equal to that at the periphery. Also, there are more electrons incident on unit area on the surface of wafer 120 (i.e. the electron current density) in the center of wafer 120 than at the periphery.
Particularly when a film to be etched on wafer 120 is not conductive, the surface of wafer 120 will be charged not uniformly because of the non-uniformity of the number of electrons which are incident on the surface of wafer 120.
FIG. 16 is a schematic sectional diagram of the wafer showing the surface of the wafer being charged up not uniformly. Referring to FIG. 16, wafer 120 includes a substrate 121, a film to be etched 122 which is deposited on the surface of substrate 121, and a resist pattern 123. Resist pattern 123 is patterned to have a desired form, and thus an exposed surface 122a in the center (a dash-dotted line c--c) of the reaction chamber and an exposed surface 122b at the periphery are exposed respectively from resist pattern 123. As described above, there are more electrons incident at exposed surface 122a than at exposed surface 122b, since the electron current density becomes heavier toward the center of wafer 120. Thus, exposed surface 122a will be charged up deeply negative than the exposed surface 122b.
Generally, ions 182 in the plasma will be incident on the surface of wafer 120 until the surface of wafer 120 is in electrically steady state, in other words, the electron current density and the ion current density become equal. Accordingly, the number of ions 182 which are incident at exposed surface 122a is greater than ions 182 which are incident at exposed surface 122b. Etching rate will be higher at exposed surface 122a than at exposed surface 122b. As a result, the amount of etching is relatively large at exposed surface 122a in the center of wafer 120 (a dash-dotted line c--c), while the amount of etching is relatively small at exposed surface 122b at the periphery as shown in FIG. 17. Thus, when the distribution of electron density becomes non-uniform, a film to be etched in the wafer may not possibly be etched uniformly.
Also, if the area of exposed surface 122a or the like is large, the amount of etching at the center (line c--c) is large and the amount of etching is small in the periphery even within one exposed surface 122a as shown in FIG. 18, possibly causing non-uniform etching.
Non-uniformity of etching might occur also when the film to be etched is conductive. FIG. 19 is a schematic sectional diagram of the wafer showing that non-uniformity of etching occurs when the film to be etched is conductive. Referring to FIG. 19, wafer 120 includes a substrate 124, an insulating film 125 which is formed at the surface of substrate 124, a conductive film 126, and a resist pattern 127 which is formed at the surface of conductive film 126. Resist pattern 127 is patterned to have a desired form and conductive film 126 will be etched using resist pattern 127 as a mask. During etching, the electron current density is higher in the center of the reaction chamber (a dash-dotted line c--c) than at its periphery. However, when the film to be etched is conductive, such as in the case of polycrystalline silicon, electrons will move in the conductive film 126. Conductive film 126 is thus charged up negatively on the entire surface of 120 uniformly. Accordingly, the number of ions incident on each exposed surface of conductive film 126 will be equal, resulting in uniform amount of etching.
However, if the conductive film 126 is overetched, underlying insulating film 125 formed of silicon oxide film or the like will be exposed. As insulating film 125 is not conductive, non-uniformity of etching might occur as described above in which the amount of etching is large at exposed surface 125a in the center of the reaction chamber (a dash-dotted line c--c) while the amount of etching of exposed surface 125b as the periphery is small.
As described above, etching on the surface of the wafer might be non-uniform when the electron density in the ECR plane becomes non-uniform.
Further, when the distribution of the electron density in the ECR plane is non-uniform, local deviation of the distribution of the electron density will occur as shown at B of the FIG. 20, which is so called drift unstability. This is described, e.g. in Motohiko Tanaka and Takaharu Nishikawa, "Physics of High Temperature Plasma" (Maruzen). When the drift unstability occurs, random movement of ions will be enhanced because ions in the plasma move along the local deviation of the electron density.
FIG. 21 shows behavior of ions in the reaction chamber when drift unstability occurs. Referring to FIG. 21, when the random movement of ions 182 is enhanced, the number of ions proceeding into an ion sheath region 150 with a small approach angle .theta. is increased. The travel direction of ions within ion sheath region 150 is determined by the sum of vectors of ions at the time of approaching into ion sheath region 150 and vectors of the ion sheath electric field E. Thus, when the approach angle .theta. is small, ions are hardly incident vertically on the surface of wafer 120.
FIG. 22 is an enlarged partial sectional view of the wafer schematically showing ions incident on the wafer. Referring to FIG. 22, wafer 120 includes a film to be etched 128 and a resist pattern 129 which is formed on film to be etched 128. Resist pattern 129 is patterned to have a desired form, and underlying film to be etched 128 will be etched using resist pattern 129 as a mask. During the etching, if an ion 182 is not incident vertically on the surface of wafer 120, then ion 182 will collide against the sidewall of a groove 128a. The sidewall of groove 128a will be etched and removed as shown by a dotted-line 128b accordingly, so that highly anisotropic etching will be difficult.
As described above, there was a problem where the anisotropic property of etching might be impaired because of unstability of drift due to a non-uniformity of the electron density.
Generally, most ions move along the flow of electrons traveling along the magnetic line of force. If the ion density in the ECR plane is not uniform as shown in FIG. 14, then the number of ions incident on wafer 120 is considered not to uniform, either. Referring to FIG. 12, the number of ions incident at the center of wafer 120 will be greater than the number of ions incident on the periphery of wafer 120, because the ion density in the center of reaction chamber 101 (a dash-dotted line c--c) is relatively large, while that is relatively small at the periphery of reaction chamber 101. Thus, the etching of wafer 120 will not be carried out uniformly.
As described above, there was a problem of non-uniform etching when the ion density in the ECR plane becomes not uniform.
When the distribution of the plasma potential becomes non-uniform as shown in FIG. 13, anisotropic property of the etching might be impaired, which will be described in detail below.
FIG. 23A is a schematic sectional view of the reaction chamber showing the reduction of anisotropic property of the etching when the distribution of the plasma potential is not uniform. FIG. 23B shows the distribution of the plasma potential in the plane indicated by a dotted-line 131 in FIG. 23A. Mainly referring to FIG. 23A, in the conventional plasma treatment apparatus, the plasma potential in the radial direction of reaction chamber 101 becomes non-uniform in general, due to the difference in mobility of electrons as well as ions. In other words, the plasma potential becomes non-uniform also in 131 plane 131 of the radial direction in addition to ECR plane 130. Ions in the plasma have positive charges, so that they tend to move to where the plasma potential is low. Thus, ion 182 which travels from the upper portion of reaction chamber 101 to wafer 120 will move from the center of reaction chamber 101 to the periphery. As a result, the approach angle .theta. of ion 182 which proceeds into ion sheath region 150 is reduced, so that its anisotropic property might be decreased as described above.
As described above, non-uniformity of the plasma potential might decrease the anisotropic property of etching.